spies will be unites that the other player thinks is his. eg player Alpha trains one of his populace into a captain, however this unit is in reality owned by player Beta, player Beta lets player Alpha use this unit as if it his own. From this player Beta gains info on troop movement. When Player Alpha places what he thinks is his captain next to one of his nobles, player Beta blows the spies cover and attempts to assassinate the noble. note: re the labs, if player Alpha uses and trains the spy unit as a scientist in a lab, player beta will secretly gain the tech researched.

I see no reason to let spies be caught before they have blown their cover. Players will of course be suspicious of all their units, probably only trusting their oldest units. They may suspect a unit as spy, and they can of course execute a unit they think is a spy, but they will not be sure. The only unit that can not be a spy is the noble, hence this will encourage more use of this unit (as a sub note- a very rare tech, shape change, allows your spy units to [if they succeed in an assassination] take the place of a unit, so they may become a noble without the target player knowing, the target will think his noble avoided an assassination and the assassin was killed). Also note, a unit you train as a spy may already be a spy! these units are referred to as double (triple...) agents and their true allegiance will be to first faction that trained them.

spies in a space ship crew can: 1) steal the ship (ship captain); 2) sabotage/destroy the ship (engineer); 3) sabotage the ships attack/defense rating (gunner/pilot)

post scriptum, I am not yet sure if the guild will get involved in spying, but if it does, it will be for financial purposes (i.e. selling its spies to other interested parties)